1. Field of the Invention
The present invention relates to a 180.degree. phase-shifting, signal-splitting device engraved on one side of an insulating or semi-insulating substrate suitable for interface with hybrid micro-electronic structures.
2. Description of the Prior Art
A "rat race", FIG. 1A, has been described as a circular loop of coaxial line closed upon itself and having four branching connections. It has essentially the same properties of what is also called electronically a magic tee. In the case of the most commonly used type of "rat race", the coaxial loop has a circumference of 1.5 wavelengths. One 3/4 wavelength spacing between connections is employed and the other three spacings are made of 1/4 wavelength in length. In a properly matched "rat race" operating at the correct frequency, several useful affects are achieved. Energy entering the rat race from arm 1 is evenly split between arms 3 and 4 and no power reaches arm 2. A wave entering the rat race at arm 3 is evenly split between arms 1 and 2 and no power reaches arm 4. To use the rat race as a transmitter/receiver duplexer, the transmitter is connected to arm 3, the antenna to arm 1 and the receiver to arm 4 and a match load to arm 2. During transmission, no energy reaches the receiver. During reception, energy from the antenna is fed to the receiver. A three dB loss occurs during both transmit and receive periods but this loss is shown to be unavoidable in any passive network duplexer. In some millimeter wavelength applications, rectangular or circular waveguides may replace coaxial lines in "rat races".
Hybrid ring "rat race" couplers have been used extensively in microwave circuit applications. However, the large size and narrow band performance of the "rat race" precluded its use in many modern microwave circuit applications especially in monolithic microwave integrated circuits (MMICs). The present invention defines a broadband, reduced size, 180.degree., three dB hybrid ring coupler suitable for both monolithic and hybrid microwave circuit applications. This coupler theoretically maintains a 180.degree. transmission phase difference and perfect isolation between the two output ports independent of the input frequency.
In June, 1968 on IEEE Transactions on Microwave Theory and Techniques Article entitled "A Wideband Strip Line Hybrid Ring" by Steven March disclosed a narrow band device, a "rat race" hybrid that can be broadbanded for good performance over an octave by incorporating several design changes. The limiting factor in the usual hybrid ring coupler is the 3/4 wavelength section which restricts the useful frequency range for the 180.degree. hybrid to F.+-.0.23F where F is the center frequency in the band of interest. A conventional ring configuration exhibits a -3 dB of coupling when the characteristic impedance of each ring segment Z is 2Z.sub.0 where Z.sub.0 is the characteristic impedance of both the input and output lines. The March device realized a decreased frequency sensitivity by replacing the 3/4 wavelength section by one having the same mid-band impedance, but whose electrical length is realized by a 1/4 wavelength of line exhibiting the characteristics of an ideal phase reversing network. A pair of equilateral broadside coupled segments of strip transmission line having diametrically opposed ends short circuited approximates a phase reversing network over a wide frequency range. The use of this configuration in a magic tee is one example. The March device is somewhat frequency insensitive. However, the length of the segments between the four ports is determined by the frequency of the input signal. If the frequency of the input signal is the determining factor in the length of each of the arcuate segments between each port, then the device is size constrained.
The problem to be solved is the production of a hybrid microelectronically compatible 180.degree. phase shifting structure which is frequency insensitive and is frequency independent whereby the frequency of the input signal does not determine the length of the arcuate segments connecting the four ports.